This topic describes the process and API endpoints you can use to extend proxies for integration with Consul.
## Overview
You can extend any proxy to support Connect. Consul ships with a built-in
proxy suitable for an out-of-the-box development experience, but you may require a more robust proxy solution for production environments.
The proxy you integrate must be able to accept inbound connections and/or establish outbound connections identified as a particular service. In some cases, either ability may be acceptable, but both are generally required to support for full sidecar functionality.
Sidecar proxies may support L4 or L7 network functionality. L4 integration is simpler and adequate for securing all traffic. L4 treats all traffic as TCP, however, so advanced routing or metrics features are not supported.
Full L7 support is built on top of L4 support. An L7 proxy integration supports most or all of the L7 traffic routing features in Connect by dynamically configuring routing, retries, and other L7 features. The built-in proxy only supports L4, while [Envoy](/docs/connect/proxies/envoy) supports the full L7 feature set.
The client certificate from the inbound connection must be validated against the Connect CA root certificates. Call the [`/v1/agent/connect/ca/roots`] endpoint to obtain the root certificates from the Connect CA, e.g.:
After validating the client certificate from the caller, the proxy can authorize the entire connection (L4) or each request (L7). Depending upon the [protocol] of the proxied service, authorization is performed either on a per-connection (L4) or per-request (L7) basis. Authentication is based on "service identity" (TLS), and is implemented at the
-> **Note:** Some features, such as (local) rate limiting or max connections, are expected to be proxy-level configurations enforced separately when authorization calls are made. Proxies can enforce the configurations based on information about request rates and other states that should already be availabe.
The proxy can authorize the connection by either calling the [`/v1/agent/connect/authorize`](/api/agent/connect) API endpoint or by querying the [intention match API](/api/connect/intentions#list-matching-intentions) endpoint.
If the local Consul agent is down or unresponsive, the success rate of new connections will be compromised.
The agent uses locally-cached data to authorize the connection and typically responds in microseconds. As a result, the TLS handshake typically spans microseconds.
~> **Note:** This endpoint is only suitable for L4 (e.g., TCP) integration. The endpoint always treats intentions with `Permissions` defined (i.e., L7 criteria) as `deny` intentions during evaluation.
The proxy can query the [intention match API](/api/connect/intentions#list-matching-intentions) endpoint on startup to retrieve a list of intentions that match the proxy destination. The matches should be stored in the native filter configuration of the proxy, such as RBAC for Envoy.
For performance and reliability reasons, querying the intention match API endpoint is the recommended method for implementing intention enforcement. The cached intentions should be consulted for each incoming connection (L4) or request (L7) to determine if the connection or request should be accepted or rejected.
For a proxied service configured with the TCP [protocol], potentially long-lived TCP connections will only be authorized when the connections are initially established. But because many services, such as databases, typically use persistent connection pools, changing intentions to deny access does not terminate existing connections. This behavior violates the updated intention. In these cases, it may appear as if the intention is not being enforced.
1. **Configure connections to terminate after a maximum lifetime**, e.g., several hours. This balances the overhead of establishing new connections with determining how long existing connections remain open after an intention changes.
1. **Periodically re-authorize every open connection**. The authorization call is inexpensive and should be a local, in-memory operation on the Consul agent. Periodically authorizing thousands of open connections (e.g., once every minute) is likely to be negligible overhead, but doing so enforces a tighter upper boundary on how long it takes to enforce intention changes without affecting the protocol efficiency of persistent connections.
Intentions currently use TLS URI Subject Alternative Name (SAN) for enforcement. The `AuthZ` API in the Go SDK contains a field for passing the serial number ([`consul/connect/tls.go`]). Proxies may provide this value during authorization.
### Updating data
The API endpoints described in this section operate on agent-local data that is updated in the
background. The leaf, roots, and intentions should be updated in the background
by the proxy.
The leaf cert, root cert, and intentions endpoints support [blocking
queries](/api/features/blocking), which should be used to get near-immediate
updates for root key rotations, new leaf certs before expiry, and intention
changes.
### SPIFFE certificates
Although Consul follows the SPIFFE spec for certificates, some CA providers do not allow strict adherence. For example, CA certificates may not have the correct trust-domain SPIFFE URI SAN for the
cluster. If SPIFFE validation is performed in the proxy, be aware that it
should be possible to opt out, otherwise certain CA providers supported by
Consul will not be compatible with the use of that proxy. Neither
Envoy nor the built-in proxy currently validate the SPIFFE URI of the chain beyond the
[`/v1/agent/connect/ca/leaf/`] API endpoint. The proxy must use the root certificate obtained from the [`/v1/agent/connect/ca/roots`] endpoint to verify the certificate served from the destination endpoint.
The [`/v1/agent/service/:service_id`](/api/agent/service#get-service-configuration)
API endpoint enables any proxy to discover proxy configurations registered with a local service. This endpoint supports hash-based blocking, which enables long-polling for changes
result in the new config being returned immediately.
Refer to the [built-in proxy](/docs/connect/proxies/built-in) for an example implementation. Using the Go SDK, the proxy calls the HTTP "pull" API via the `watch` package: [`consul/connect/proxy/config.go`].
Proxies can use Consul's [service discovery API](`/v1/health/connect/:service_id`) to return all available, Connect-capable endpoints for a given service. This endpoint supports a `cached` query parameter, which uses [agent caching](/api/features/caching) to improve
performance. The API package provides a [`UseCache`] query option to leverage caching.
In addition to performance improvements, using the cache makes the mesh more resilient to Consul server outages. This is because the mesh "fails static" with the last known set of service instances still used, rather than errors on new connections.
Upstreams may be defined with the Prepared Query target type. These upstreams
should use Consul's [prepared query](/api/query) API to determine a list of upstream endpoints for the service. Note that the `PreparedQuery` API does not support blocking, so proxies choosing to populate endpoints in memory will need to poll the endpoint at a suitable and, ideally, configurable frequency.
-> **Long-term support for [`service-resolver`](/docs/connect/config-entries/service-resolver) configuration
entries**. The `service-resolver` configuration will completely replace prepared queries in future versions of Consul. In some instances, however, prepared queries are still used.
The proxy service ID comes from the user. See [`consul connect envoy`](/commands/connect/envoy#examples) for an example. You can use the `-proxy-id` flag to specify the ID of the proxy service you have already registered with the local agent.
Alternatively, you can start the service using the `-sidecar-for=<service>` option. This option queries Consul for a proxy that is registered as a sidecar for the specified `<service>`. If exactly one service associated with the proxy is returned, the ID will be used to start the proxy. Your controller only needs to accept `-proxy-id` as an argument; the Consul CLI will resolve the
